As the miniaturization of synthesized functional structures that possess electrical, optical and mechanical functionalities continues to progress rapidly, fabrication techniques based on conventional multi-steps of photolithography and subsequent etching processes appear to be approaching to their practical limits quickly. In the quest for discovering alternative techniques to such “top-down” approaches in which bulk materials are engraved into small-scale functional structures, the concept based on “bottom-up” approaches in which small-scale functional structures are formed by spatially arranging nanoscale building blocks, e.g. atoms and/or molecules, on given foreign substrates have been gaining significant attentions.
One of the bottom-up approaches that have been explored extensively in the past ten years is spontaneous self-assembled quantum dot, in particular, coherent, i.e. free from structural defects, small semiconductor inclusions, with a linear order of several tenths of nanometers, in a semiconductor matrix. However, inherent challenges associated with various formation techniques of spontaneous self-assembled quantum dots (SAQDs) have been hindering them from being prosperous approaches to synthesize small-scale functional structures.
As the term “spontaneous” indicates, the lack of control on specifically positioning SAQDs into densely-packed multi-dimensional periodic arrays has been a serious issue that needs to be aggressively addressed to ensure flexible tuning of physical properties of the small-scale functional structures consisting of SAQDs. One of diverse approaches that result in arranging SAQDs into periodic arrays, to some extent, as in prior art [T. I. Kamins and R. S. Williams, Appl. Phys. Lett. 71, 1201 (1997)] schematically shown in FIGS. 1A-1C, is to use engineered strain field generated by pre-formed three-dimensional structures. In FIG. 1A, a starting substrate 1 is a standard substrate having flat surface. Then, as in FIG. 1B, the starting substrate 1 is pre-patterned to create a mesa structure 2. Appropriately designed three-dimensional geometry of the mesa structure 2 provide narrow regions where the formation of SAQDs 3 is energetically favorable, thus SAQDs 3 form along the mesa top as in FIG. 1C. Although this approach can result in positioning SAQDs 3 in a particular geometrical arrangement, this does not seem to be a feasible way to obtain a densely-packed array because the formation of SAQDs 3 is spatially limited within small regions around the generated strain field. In another prior art (not shown here), an energetic beam consisting of charged particles such as ions can be used to create regions where the formation of SAQDs is energetically enhanced, however, this process would be time-consuming and/or very expensive.
Another intrinsic limitation in the formation of SAQDs relates to single crystal substrates on which SAQDs are, in most of cases, formed. Since semiconductor SAQDs are formed under the influence of mechanical strain generated by physical mismatches between a SAQDs material and a substrate material, the substrate necessarily need to be single crystal, putting substantial limitations in terms of choosing substrate materials.
On the other hand, organic nanoscale templates for the formation of arrays of nanoscale building blocks are being developed using both artificial and natural materials such as block copolymers, DNA, bacteria, virus, phage and proteins, all of which, unlike semiconductor SAQDs, have a built-in capability of arranging their organic nanoscale building blocks into two-dimensional arrays on a wide range of substrates. These organic nanoscale templates can apparently be used to arrange foreign inorganic nanoscale building blocks, e.g. semiconductor QDs, into two-dimensional arrays characterized by the original organic nanoscale templates. However, as in a prior art [R. A. Mcmillan, et al, “Ordered nanoparticle arrays formed on engineered chaperonin protein templates”, Nature Materials 1, 247 (2002).], general incompatibilities in physical properties of such organic nanoscale templates when incorporated as a part of functional device consisting of arrayed inorganic nanoscale building blocks clearly indicate that organic nanoscale templates eventually need to be removed.
Therefore, it would be desirable to have a capability of transferring an array of nanoscale building blocks from an original substrate on which the array is preferably formed using a nanoscale template to another substrate on which only the array of nanoscale building blocks resides eventually. It would be further desirable to have a capability of arranging many arrays of nanoscale building blocks into three-dimensional structures. These are necessary to fabricate the novel devices (optical and electrical) having significantly high performances as compared with the bulk-based or non-uniform quantum dot based devices.